Measurement optics in a polarization based or multiplexed heterodyne interferometer such as used for precision measurements in semiconductor device manufacturing equipment generally require a light beam including orthogonal polarization components that have slightly different frequencies. However, the light sources such as lasers that generate the required beams also produce heat and temperature gradients that are generally unacceptable for precision operation of the interferometer measurement optics. Accordingly, the light sources generally must be separated from the measurement optics. Transmission of the polarization components on a single optical fiber is generally not used because even a polarization-maintaining (PM) fiber will typically change the polarizations too much for the precise measurements.
In one system for beam delivery, a beam splitter feeds one polarization component to a first PM fiber and feeds the second polarization component to a second PM fiber. The two separate fibers carry the two component beams to the interferometer optics near the measurement site. Polarizers, alignment optics, and a combiner can then recombine the two component beams so that the beams travel along a common path. Suitable alignment optics include, for example, collimators, flat transmissive windows, polarizing optics, beam splitters, and total and partial reflectors to translate and tilt the beams. The interferometer optics then reflect one of the component beams off of a stage being measured and reflect the other component off of a reference reflector. The phase difference between the two beams after respective reflections indicates the displacement or movement of the stage.
Conventional interferometer systems using two PM fibers for beam delivery often require sensitive alignment processes for the optics and combiner that recombine the two component beams. Additionally, having two fiber paths generally requires duplication of equipment or optical elements, which can increase system cost and size. Thus, a system that maintains the polarization of two light beams in a single fiber could reduce cost and complexity.